1. Field of the Invention
The present invention relates to a projection exposure apparatus for forming a fine resist pattern required for the manufacture of a semiconductor integrated circuit and a pattern formation method using the apparatus and, more particularly, to a projection exposure apparatus in which an optical system is optimized to maximize the depth of focus.
2. Description of the Related Art
With the recent remarkable advances in photolithography, it may be possible to realize a "0.5-.mu.m rule" by a projection exposure apparatus using a g-line (436 nm) or an i-line (365 nm). Such expectation is based on the recent improvement in the performance of a projection exposure apparatus, especially an increase in the NA of a lens. It is, however, doubtful that a "0.3-.mu.m rule" of the next generation can be achieved by the extension of the conventional techniques. It is true that an increase in resolving power can be achieved by increasing the NA of a lens or reducing the wavelength of exposure light. However, the depth of focus is decreased, and hence the practical resolving power is not increased as expected. Therefore, demands have currently arisen for the development of a technique of achieving an increase in depth of focus.
FIG. 1 shows the basic schematic arrangement of a projection exposure apparatus which has been generally used. As shown in FIG. 1, the projection exposure apparatus comprises a lamp 1 constituted by a mercury-vapor lamp, an elliptic reflection mirror 2 whose second focal point is located at point 3, an input lens 4, an optical integrator (fly-eye lenses) 5, an output lens 6, a collimation lens 7, a reticle (or mask) 8, an aperture stop 9 as a uniform stop, a filter 10 which serves so that the optical system transmits aberration-corrected light, cold mirrors 11 and 12 for bending the optical path to reduce the height of the apparatus, a lamp house 13, a projection optical system 14 for projecting a pattern image, formed on the reticle (mask) 8, onto a wafer by means of lenses, mirrors, or a combination thereof, a wafer 15, and a stop 16 for determining a numerical aperture.
Although there are various basic arrangements of conventional projection exposure apparatuses, in practice, in addition to the one shown in FIG. 1, they can be generally expressed, as shown in FIG. 2A. That is, in a projection exposure apparatus of this type, a light source 1, a first focusing optical system 18, a uniforming optical system 19, a second focusing optical system 20, a reticle 8, a projection optical system 14, and a wafer 15 are arranged in the order named.
The first focusing optical system 18 corresponds to the elliptic reflecting mirror 2 and the input lens 4 in FIG. 1. In the first focusing optical system 18, a spherical mirror, a plane mirror, a lens, and the like are optimally arranged in addition to the elliptic mirrors so as to cause beams emitted from the light source to be incident on the uniforming optical system 19 as efficiently as possible.
The uniforming optical system 19 corresponds to the optical integrator 5 in FIG. 1. The uniforming optical system 19 may also include, for example, an optical fiber or a polygonal prism in addition to the optical integrator.
The second focusing optical system 20 corresponds to the output lens 6 and the collimation lens 7 in FIG. 1. The optical system 20 serves to superpose beams, emerging from the uniforming optical system 19, on each other, and ensures image plane telecentricity. In addition, a filter corresponding to the filter 10 in FIG. 7 is inserted in the optical system 20 at a position close to the optical axis. Furthermore, reflecting mirrors corresponding to the cold mirrors 11 and 12 are inserted (their positions are not uniquely determined).
In the projection exposure apparatus as shown in FIG. 2A having the above-described arrangement, since the characteristics of light emerging from the reticle 8 are the same as those of light emerging from the uniforming optical system 19 through the second focusing optical system 20, the exit side of the uniforming optical system 19 may serve as a so-called apparent light source. In the above arrangement, therefore, an exit side 24 of the uniforming optical system 19 is generally called a secondary source. In addition, when an image of the reticle 8 is projected on the wafer 15, the formation characteristics (i.e., resolving power, depth of focus, and the like) of a projected/exposed pattern are determined by the numerical aperture of the projection optical system 14 and the characteristics of light (i.e., the characteristics of the secondary source 24) radiated on the reticle 8.
FIG. 2B is a view for explaining reticle illumination light and focused light in the projection exposure apparatus shown in FIG. 2A. Referring to FIG. 2B, a projection optical system 14 generally includes an aperture stop 16 to regulate a predetermined angle ea at which light passing through a reticle 8 can pass therethrough and which determine an angle .THETA. at which light is incident on a wafer 15.
The numerical aperture (NA) of a projection optical system is generally defined as NA=sin.THETA.. If the projection magnification is 1/m, sin.THETA.a=sin.THETA./m. Furthermore, an exposure apparatus of this type generally has a so-called "image plane telecentric" arrangement (i.e., a principal ray is perpendicularly incident on an image plane). In order to satisfy the condition for this "image plane telecentric arrangement, a real image from the exit surface of the uniforming optical system 19 (i.e., the light source surface of the secondary source 24) shown in FIG. 2A is focused at the position of the aperture stop 16.
Under the above-described conditions, provided that an elevation angle at which the secondary source is viewed from the reticle 8 through the second focusing optical system 20 is obtained within the range of light incident on the reticle 8, half of the elevation angle is assumed to be .PHI., and that a coherence .sigma.=sin.PHI./sin.THETA.a, it is considered that the pattern formation characteristics are determined by NA .sigma..
The relationship between NA, .sigma., and pattern formation characteristics will be described in detail next. With an increase in NA, the resolving power of a projection exposure apparatus is increased. On the other hand, the depth of focus is decreased, and a wide exposure field is difficult to ensure because of the aberration of the projection optical system 14. Such a projection exposure apparatus cannot be used for the actual manufacture of an LSI or the like unless it has a certain exposure region and depth of focus (e.g., 20.times.20 mm .+-.1 .mu.m). Therefore, in a conventional apparatus, the limit value of NA is about 0.55. The .sigma. value is mainly associated with the cross-sectional shape of a pattern and the depth of focus, and is associated with the resolving power with correlation with the cross-sectional pattern shape. With a decrease in the .sigma. value, the edge of a pattern is emphasized so that the angle of the side wall of the cross-sectional shape approaches a rectangular-shape, thus obtaining a good pattern shape. On the other hand, the resolution of fine patterns deteriorates, and the focal range within which the patterns can be resolved is narrowed. In contrast to this, with an increase in the .sigma. value, the resolution of fine patterns is slightly improved, and the focal range is slightly widened. However, the inclination (angle) of the side wall of the pattern is reduced. For this reason, if, for example, a thick resist is formed, its cross-sectional shape becomes trapezoidal or triangular. In the conventional projection exposure apparatus, therefore, .sigma.=0.5 to 0.7 is fixed as the .sigma. value for achieving a relatively good balance between these merits and demerits. Pattern formation under the condition of .sigma.=0.3 or the like is only performed as an experiment. In addition, since the G value can be set by determining the size of the light source surface of the secondary source 24, the circular aperture stop 9 for setting the .sigma. value is placed immediately after the light source surface of the secondary source 24.
An annular illumination exposure technique or method (for example: Published Unexamined Japanese Patent Application No. 61-91662) is one of the methods of increasing the depth of focus of a general projection exposure apparatus such as the one described above. In this method, a special stop is inserted in a projection exposure apparatus such that the intensity of a peripheral portion is higher than that of a central portion in the intensity distribution of the secondary source of the apparatus. More specifically, one of the different types of filters shown in FIGS. 3A to 3D is inserted in the apparatus in place of the a value setting circular aperture stop 9 shown in FIG. 1. FIG. 3E shows a transmittance characteristic of the filter shown in FIG. 3D. As an example, the depth-of-focus increasing effect obtained by the filter shown in FIG. 3A was evaluated by simulation. The result is shown in the graph in FIG. 4, in which the ordinate represents the depth of focus with respect to the mask pattern size, which value corresponds to a central shielding ratio .epsilon. (i.e., pattern size dependence) of an annular illumination filter. This central shielding ratio is represented by .epsilon.=r2/r1 where r1 and r2 are the outer and inner diameters of each of the annular filters shown in FIGS. 3A and 3C. In addition, the exposure apparatus has an NA of 0.54, a coherence factor of 0.5, and an exposure wavelength of 436 nm (g-line). It is apparent from these characteristic curves that the limit resolving power and the depth of focus are increased with an increase in the .epsilon. value.
However, the actual secondary source is formed on the exit surface of the above-mentioned fly-eye lens and is spatially discrete. For this reason, if the .epsilon. value is increased too much (i.e., the annulus is narrowed), light cannot pass through the annular stop. Therefore, the .epsilon. value is normally set to be about 0.5 to 0.7. As a result, as is apparent from FIG. 4, even the maximum depth-of-focus increasing effect is as small as about 13%. At the same time, FIG. 4 indicates that the pattern size dependence of the depth-of-focus increasing effect is considerably high. That is, a certain depth-of-focus increasing effect can be recognized within the pattern size range from 0.6 to 0.9 .mu.m, but the depth of focus decreases from a pattern size of 0.9 .mu.m or more contrary to the intended effect. With regard to the annular stop shown in FIG. 3D, there has been no detailed description as to a method of determining the transmittance.
The results obtained by actually applying annular illumination exposure to pattern (e.g. Isolated line, Isolated space, etc.) formation, and L/S ratio will be described below. FIGS. 5A and 5B show a light source modulating filter actually used. An annular shielding ratio .epsilon. of the filter is defined as .epsilon.=r2/r1. The graph shown in FIG. 6 indicates the distribution of depths of focus obtained by combinations of coherences .sigma. and annular shielding ratios .epsilon. with respect to an L/S pattern having a line/space ratio of 1:1 (0.6 .mu.m). Similarly, the graph shown in FIG. 7 indicates a case of an isolated positive pattern (i.e., isolated line pattern), which has a pattern size of 0.6 .mu.m. The graph in FIG. 8 indicates a case of an isolated negative pattern (i.e., isolated space pattern), which has a pattern size of 0.6 .mu.m. The depths of focus (DOF values) shown in FIGS. 6 to 8 are obtained with a g-line, NA=0.54, a positive resist (PFR-GX200), and a film thickness of 1.0 .mu.m. Each DOF value indicates the coherence .sigma. and the annular shielding ratio .epsilon. (i.e, pattern size dependence) obtained on the basis of a contrast (critical image contrast) required for pattern formation, which contrast is calculated from a focus margin obtained in an experiment on an exposure process with coherence .sigma.=0.5 and annular shielding ratio .epsilon.=0.
In consideration of the fact that the focus margin required for an actual (pattern formation) process is about 2 .mu.m, exposure must be performed by annular illumination in the existing circumstance because limitations based on an L/S pattern are strictest, as is apparent from FIG. 6. In addition, the following conclusions can be drawn from the graphs shown in FIGS. 6 to 8.
(1) For an L/S pattern, .sigma.=0.5 to 0.7 and .epsilon.=0.6 to 0.8 are the optimal values.
(2) For an isolated positive pattern, .sigma.=0.6 to 0.7 and .epsilon. to 0.6 to 0.8 are the optical values.
(3) For an isolated negative pattern, the optimal values of .sigma. and .epsilon. are small (i.e., values for normal exposure).
As described above, the problem of such a technique is that a given optimal filter must be replaced with another optimal film depending on whether a layer constituted by an L/S pattern or an isolated positive pattern is transferred in actual exposure or a layer constituted by an isolated negative pattern and contact holes is transferred. In practice, therefore, the operation efficiency of this technique is low.
As another example, a filter having four apertures shown in FIG. 9B (to be referred to as a four-eye filter hereinafter) has been proposed (Published Examined Japanese Patent Application No. 56-9010). Projection exposure using this four-eye filter was reported in Kamon, Miyata, et al., "Reduction Projection Exposure Methods (I) and (II) using Modified Light Source", (lecture numbers 12a-ZF-3 and 12a-ZF-4) in the 52th applied physics meeting held in October of 1991. According to this report, high resolution performance with respect to L/S can be obtained not only in one direction but also in a direction perpendicular thereto.
The graph in FIG. 10 indicates transfer characteristics obtained by simulating exposure processes by illumination using the four-eye filter and by normal illumination. Referring to FIG. 10, the abscissa represents the L/S pattern size with the line and space ratio=1:1; and the ordinate, the depth of focus (DOF). The exposure wavelength is 365 nm (i-line); and the NA of a projection optical system, 0.55. In this case, it is assumed that a resist which can resolve a pattern with an image contrast of 70% or more is used. This graph shows characteristics obtained when the four-eye filter and the direction of each L/S pattern have the relationship shown in FIG. 11. The following conclusions can be drawn from this graph. With the above-described L/S pattern, the resolving power and the depth of focus are greatly increased near L/S&lt;0.65 .mu.m, especially L/S=0.4 .mu.m, by four-eye illumination as compared with normal illumination. However, when L/S.gtoreq.0.65 .mu.m, the depth of focus obtained by normal illumination is larger than that obtained by four-eye illumination. Especially near L/S=0.7 .mu.m, the depth of focus obtained by four-eye illumination is minimized. Such characteristics are dependent on the positions and sizes of the eyes (holes) of the four-eye filter. For example, with an increase in distance between the eyes, the depth of focus with a small L/S value is increased, while the depth of focus with a large L/S value is considerably decreased. In addition, with a decrease in size of each eye, the depth of focus of only an L/S pattern of a specific size tends to increase.
Although there is a slight difference in characteristic depending on the positions and sizes of the eyes of the four-eye filter, as described above, this graph shows the overall characteristics.
The above description is associated with an L/S pattern. It is found that the use of the four-eye filter adversely affects the formation of an isolated negative pattern (i.e., a positive resist), and the DOF is decreased. More specifically, the minimum space width which can ensure a depth of focus of 1.5 .mu.m is 0.5 .mu.m in normal exposure but is 0.45.mu. in exposure by means of four-eye illumination. That is, when exposure is to be performed by four-eye illumination, an L/S pattern with an L/S ratio of 1:1 can be designed on the order of 0.29 .mu.m, whereas an isolated negative pattern needs to be designed on the order of 0.45 .mu.m or more. Actual LSI pattern include only a small number of typical isolated negative patterns, each having a negative line width close to a design rule and resist portions extending on both sides by several .mu.m, but includes a large number of patterns designed such that the space to line ratio is low. Therefore, in the case of such an isolated negative pattern, the lien width which can ensure a depth of focus of 1.5 .mu.m or more is increased, greatly influencing a reduction in the size of a chip.
In four-eye exposure using a four-eye filter, it is found that good resolution performance cannot be obtained with respect to patterns arranged in directions other than two orthogonal directions. Especially for patterns arranged at an angle of 45.degree. the four-eye filter and the direction of each L/S pattern have the relationship shown in FIG. 12, the resolution performance considerably deteriorates. The characteristics shown in FIG. 13 can be obtained. In this case, it is apparent that transfer by four-eye illumination is inferior in depth of focus to transfer by normal illumination.
A method called the "superflex method" was proposed in the "Thirty-eighth applied physics joint meeting papers" (29a-Zc-8) as another method of increasing the depth of focus of a projection exposure apparatus. In this method, a spatial frequency filter is inserted in the pupil of the projection exposure apparatus. According to this reference, when the amplitudes of two images having phases shifted from each other by .+-..DELTA. and focused at different positions z=.+-..beta. in the direction of an optical axis are synthesized, the following two effects can be simultaneously obtained:
(1) an increase in depth of focus owing to a multiple focusing (FLEX) effect; and
(2) a pseudo-phase shift effect upon cancellation of phases at a pattern edge.
If an amplitude distribution in a lateral (x) direction and optical axis (z) direction is represented by U(x,z), the synthesized amplitude can be given by the following equation: EQU U.sup.1 (x,z)=[exp(i.DELTA..phi.)U(x,z+.beta.)]/2
This amplitude is equal to that obtained by arranging a spatial filter having a radial distribution of complex amplitude transmittances represented by the following equation at a projection lens pupil (aperture): EQU t(r)=cos(2C.pi..beta..sup.2 -.theta.2) (for.theta.=2.DELTA..phi.-8.pi./NA.sup.2)
If .beta. and .THETA. are properly selected, the distance between two focused images and the interference effect (or spatial frequency transfer characteristics) therebetween can be arbitrarily controlled.
In this method, however, since a filter having a small transmittance is inserted in the pupil inside a projection optical system, the filter absorbs exposure light to generate heat, and thermal expansion of the optical system occurs. As a result, the pattern transfer precision deteriorates.
In addition, the dependence of the depth-of-focus increasing effect on pattern size is high, as indicated by the graph in FIG. 14. In this graph, the abscissa represents the mask pattern size normalized by .lambda./NA; and the ordinate, the focus margin normalized by .lambda./NA.sup.2. When focus margins which ensure a pattern contrast of 60% or more on an image plane are viewed in this graph, it is found that the focus margin in the normal exposure method is larger than that in the "super flex method" with a pattern size of 0.8 (normalized size) or more.
As described above, the methods of increasing the depth of focus (i.e., the "annular illumination exposure method" four-eye illumination exposure method and the "superflex method") have the following problems, respectively).
In the "annular illumination exposure method (A)" the following four problems are posed:
(A-1) In consideration of the arrangement of an actual secondary source, the depth-of-focus increasing effect is small (13% at best).
(A-2) The dependence of the depth-of-focus effect on pattern size is high.
(A-3) With regard to an "annular stop" having a transmittance distribution, there has been no detailed description as to a method of determining the transmittance.
(A-4) The effect of the depth-of-focus enhancement is low, in case of the pattern of isolated line and contact hole.
In the "superflex method (B)" the following two problems are posed:
(B-1) The filter absorbs exposure light to generate heat, and thermal expansion of the optical system occurs, resulting in a deterioration in transfer precision.
(B-2) The dependence of the depth-of-focus effect on pattern size is high as shown in FIG. 14.
The following problems must be solved, especially in the case of four-eye illumination exposure method.
As described, in an exposure process using the four-eye filter, as compared with a normal exposure process using a circular secondary source, the resolving power and the depth of focus are considerably increased when L/S&lt;0.65 .mu.m. In contrast to this, when L/S.gtoreq.0.65 .mu.m, the depth of focus obtained by the normal illumination method becomes larger than that obtained by the four-eye illumination method (especially near L/S=0.7 .mu.m, the depth of focus obtained by the four-eye illumination method becomes as small as about 1.5 .mu.m). For this reason, when a layer demanding a large depth of focus is transferred, a pattern having an L/S near 0.7 .mu.m cannot be properly transferred (i.e., in spite of the fact that a depth of focus of 2.5 .mu.m is obtained near L/S=0.4 .mu.m, the transfer characteristics are rate-determined by a pattern having a larger size).
If a filter which transmits light only at four specific positions, such as a four-eye filter, is used, the resolution performance greatly varies depending on the direction of each pattern.
In the above-described four-eye illumination method, although the depth of focus and resolving power of an L/S pattern are increased, those of an isolated negative pattern are decreased. This indicates that a pattern must be designed on the order of a larger line width with an increase in the line width as compared with the space width of the pattern. Since actual LSI patterns include a large number of patterns having a larger line width then the space width, the great depth-of-focus increasing effect using on four-eye illumination cannot be associated with a reduction in chip size.